I agree that R1b is far too widespread crossing many different environments, economies, cultures etc for selective advantage to be a ready explanation. Valencia and north-east Scotland could not be much more different in these respects but both are very high in R1b if memory serves me right.

I have to disagree that agricultural spread of R1b from SE to NW can be ruled out by modern distribution. The modern distribution is indeed the reverse of what you would expect and that had long put me off the option. However, there is no question that there seems to be a roughly south-east to west distribution moving from ancestral to younger in terms of R1b. The presence of ancestral forms of R1b in the SE of Europe/Asia Minor puts it in a good position to have been part of the Neolithic expansion which was clearly SE to west. Perhaps it came along with the other haplogroups more commonly attributed to this (J, E clades etc). Both the LBK and Cardial/Impresso cultures seems to have spread explosively and in the latter case by leap frog colonisation. Hungary and the Adriatic Balkans coast are the likely areas of immediate dispersal westward although with roots further south and south-east. It it also thought by many that they tended to
avoid the areas already significantly settled by hunter gatherers, at least in the primary phases. The explosive nature of these may have favoured a sequence of founder effects that may have led to R1b dominating among the settlers in the west.

The current distribution in western Europe may simply be a result of the areas where the Neolithic populations have been least disturbed by subsequent migration in later prehistoric and historic times. There is undoubtedly some correlation between linguistically/ culturally conservative areas and high R1b in western Europe. However, the isolation of these areas may have begun from a starting point in the Neolithic rather than the Palaeolithic or Mesolithic.

Finally the point about most of Spain being empty at the start of the Neolithic is important. If the area had been evacuated by the hunter gatherers by then, then the present population must be Neolithic or later. What is the predominant group in this area? R1b. Is there any other country in continental Europe with as high R1b? Not as far as I know. Is there a link between this and the Neolithic settlers finding a largely empty land? You've got to consider it. One other interesting fact is that Scandinavia is thought to have had a particularly high hunter-gatherer survival and low and late Neolithic input and also has a relatively low R1b. In the British Isles there was in contrast a very abrupt change to Neolithic culture with no evidence of prior contact and little convincing Mesolithic contribution/admixture/evolution into the Neolithic culture, almost as though there was a clear break with the past. There is also a very high R1b
count. I think the case for a Neolithic entry for R1b is stronger than it looks at first.
Alan

Dienekes said

Haplotype diversity is only one of the factors that we need to
consider when trying to figure out where and when lineages originated.

In the case of R-M269 in Europe, its distribution is fairly wide. Such
a wide distribution can be explained by:

(a) greater antiquity, i.e., more time to spread from the point(s) it
first entered Europe¬
(b) selective advantage
(c) demographic advantage, if e.g., the carriers of R-M269 had some
cultural or technological edge that boosted their numbers.

It is difficult to think of (b) as an adequate explanation, although
who knows what new evidence might turn up. It's difficult to think of
any selection factor that would operate in the widely scattered
regions of high R-M269 frequency but not elsewhere in West Eurasia.

(c) is possible, but the only two processes that could explain the
wide distribution of R-M269 are the adoption of the Neolithic economy,
or the spread of Indo-European languages (if one accepts that these
two were unrelated events). These are really the only two
_pan-European_ processes of sufficient importance.

If R-M269 was linked to the dispersal of the Neolithic economy, then
it would not have the cline that it does, where populations further
away from the centers of domestication have higher frequencies of
R-M269 than those closer to them.

We can only link it to the spread of IE languages if we accept an
improbable western European IE homeland; alternatively, we have to
explain why the other frequent Y-haplogroups found in more traditional
IE homeland theories did not accompany R-M269 to western Europe.

It seems to me that (a) emerges as a better explanation than (b) or
(c). Of course, R-M269 might have moved within Europe at various time
periods, or its Anatolian variety might have accompanied other
"Neolithic" haplogroups into Europe, but it is really difficult to
come up with a plausible scenario for a Neolithic (or later)
introduction of R-M269 into Europe.

So, I would say that R-M269 is very likely to be one of the lineages
that survived in Europe during the Ice Age to repopulate it when the
climate warmed.